Univention Corporate Server 4.0 erratum 381 (bugfix)

This update addresses the following issues:
* The ACL check in the sysvol sync script is now limited
to the Policies directory.
* There are two new UCR variables samba4/sysvol/sync/from_upstream
and samba4/sysvol/sync/from_downstream which can be used to deactivate
copying files from other DCs.
* The sysvol-sync script now checks if any changes need to be
synchronized at all and it uses file locking to coordinate
concurrent read and write processes.